High performance, high speed diesel engines are often equipped with turbochargers to increase power density over a wider engine operating range, and EGR systems to reduce the production of NOx emissions.
Turbochargers use a portion of the exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers. The larger mass of air can be burned with a larger quantity of fuel, thereby resulting in increased power and torque as compared to naturally aspirated engines.
A typical turbocharger consists of a compressor and turbine coupled by a common shaft. The exhaust gas drives the turbine which drives the compressor which, in turn, compresses ambient air and directs it into the intake manifold. Variable geometry turbochargers (VGT) allow the intake airflow to be optimized over a range of engine speeds. This is accomplished by changing the angle of the inlet guide vanes on the turbine stator. An optimal position for the inlet guide vanes is determined from a combination of desired torque response, fuel economy, and emissions requirements.
EGR systems are used to reduce NOx emissions by increasing the dilution fraction (F.sub.1) in the intake manifold. EGR is typically accomplished with an EGR valve that connects the intake manifold and the exhaust manifold. In the cylinders, the recirculated exhaust gas acts as an inert gas, thus lowering the flame and in-cylinder gas temperature and, hence, decreasing the formation of NOx. On the other hand, the recirculated exhaust gas displaces fresh air and reduces the air-to-fuel ratio (AFR) of the in-cylinder mixture.
Visible smoke can be avoided by maintaining the AFR sufficiently lean, while low NOx (emissions is achieved by keeping F.sub.1 sufficiently large. Consequently, the performance of an engine control strategy is evaluated in terms of its ability to regulate AFR and F.sub.1. Neither of these performance variables, however, is directly measured. Thus, conventional control schemes generate control signals for EGR and VGT actuators to enforce tracking of set points on measured variables--typically intake manifold pressure P.sub.1 (measured by a manifold absolute pressure (MAP) sensor) and compressor mass airflow W.sub.a (measured by a mass airflow sensor (MAF)). The desired set points are typically achieved by independently controlling the VGT to regulate P.sub.1 and the EGR to regulate W.sub.a. This can result in large actuator effort to enforce the tracking of the measured variables. Consequently, there exists a need for a robust engine control strategy having stable regulation of the AFR and F.sub.1 which coordinates the control of the EGR and VGT.